Printed circuit assemblies (“PCAs”) are used in many types of electronic products. A PCA typically has several electronic components mounted on a printed circuit board (“PCB”), which is also often referred to as a printed wiring board. The types of electronic components used in PCAs include individual components, such as packaged capacitors, chip capacitors, inductors, individually packaged transistors, integrated circuits (“ICs”), and many others.
The electronic components are usually soldered to the PCB, which provides secure mechanical and electrical connections. Metal layers on the printed circuit board are patterned to interconnect the electronic components to each other and to interfaces, such as sockets, plugs, and cables, which are then connected to other PCAs or other devices. Some components have leads that extend into plated holes in the circuit board, which are then soldered in place. Using soldered leads is particularly desirable for components that undergo high stress loads, such as plugs, sockets, and large ICs.
Many large ICs are packaged in lead-frames that have leads around the perimeter of the device. The leads are soldered to corresponding solder pads or inserted into corresponding plated holes of the printed circuit board and soldered in place. The leads are relatively compliant and reduce the stress loads on the solder joints arising from shock and vibration of the PCA. However, perimeter leads limit the placement of connections on the PCB, and frequently of the IC, to the perimeter of the device. Similarly, the number of leads around the perimeter of a device increases in proportion to the perimeter, whereas the functionality of the IC, and hence the need for additional connections, often increases as the square of the device area.
Ball-grid array (“BGA”)-type ICs, which will be referred to as simply “BGAs,” have an array of solder balls on the bottom of an IC, which provide electrical connections to the IC. The solder balls on the bottom of BGA are aligned with corresponding solder pads or solder bumps on a PCB, and the solder balls are reflowed. BGAs are desirable because they can provide a very large number of electrical connections to the IC, and the number of connections scales with the area of the IC. BGAs are also desirable because electrical connections can be made to the interior of the IC without having to come through the perimeter of the IC, which simplifies trace routing in many IC designs.
However, the solder balls are relatively short, stiff, and weak compared to the perimeter leads used with other packaged ICs, such as quad packs and dual in-line packages (“DIPs”). Even though there are usually many solder balls in a BGA device, solder joint may fail in some instances. In particular, if a PCB with a BGA device is subjected to large shock loads, vibration, or distortion (such as twisting or relative contraction/expansion of the printed circuit board due to thermal or mechanical stress), a reflowed solder ball might crack or detach from the BGA or the PCB, causing an open circuit. All the reflowed solder ball connections have failed in some cases, and the BGA has detached from the PCB.
Various techniques have been used to try and improve the reliability of BGAs mounted on PCAs. One approach is to add adhesive under the BGA after it is attached to a PCB. The adhesive is typically applied in a fluid state and cured by heating to a temperature below the melting point of the BGA solder. While the adhesive adds strength to the BGA-PCB bond, the adhesive is not very stiff or strong. Application of the adhesive is relatively messy and subject to process variations. Rework (i.e. removing the BGA and the cured adhesive from the PCB and replacing the BGA) is time-consuming, and in many instances is not possible.
Another approach is to pre-load the BGA with stress in a selected direction by securing the BGA to an external brace, such as a pedestal that is cast or machined into a metal shield, or a tab or flap folded out of a sheet metal brace. A gap pad is typically installed between the pedestal and the BGA to help conduct heat away from the BGA. Most vibration loads will not exceed the preload force, and so the BGA will not lift off of the external brace in vibration. However, this approach uses a stiff structure spanning the PCA, which is itself stiff, adding weight, cost, thickness, and complexity to the overall assembly. Additionally, this method does not guard against heavy shock loads, such as an end-use type pulse (e.g. 125 G's, 2 msec, half-sine), since the amount of preload that is required to protect against this type of shock may cause unacceptable board deflection.
Another approach as been to reduce the span of a PCB by providing machined or cast shields. For example, a shield has walls surrounding a BGA that are attached to the PCB with screws. Alternatively, a shield on one side of the PCB is attached to a shield on the opposite side of the PCB. The shield walls reduce the span over which the PCB can deflect, decreasing stress on the solder joints of the BGA. However, the shields add mass, cost, complexity, and thickness to the total assembly, and the walls of the shield(s) take up area on one or both outside layers of the PCB. The walls of the shield generally extend along an outer surface of the PCB, which affects the electrical characteristics of surface traces (i.e. traces in the first metal layer of the PCB) that pass beneath the wall. This reduces the amount of first metal layer area available for trace routing, and complicates routing signals beneath the walls of the shield.
Therefore, techniques for reducing the bending stress at BGAs on PCBs that avoid the problems discussed above are desirable.